On some systems there is a valve that needs to be closed so that pumped water does not recirculate.
I had one like that, and the water would pump out of the pump, but it would just run back into the water tank through this valve.
The purpose of the valve was to allow filling the tank from the connected shore hose if the valve was open. If the valve was closed, outside shore water would just run to the taps.
Mike
2012 Winnebago Impulse Silver 26QP
2005 16.6 Double Eagle
2018 Jeep Wrangler JK
previously Snowbird Campers,
Triple E Motorhome and Fifth Wheel